There is a great need in industries such as the semiconductor industry for sensitive metrology equipment that can provide high resolution and non-contact evaluation capabilities, particularly as the geometries of devices in these industries continue to shrink. Manufacturers have increasingly turned to optical metrology techniques, such as ellipsometry and reflectometry, which typically operate by illuminating a sample with a probe beam of electromagnetic radiation and then detecting and analyzing the reflected and/or transmitted energy. The probe beam can consist of polarized or unpolarized radiation, and can include one or more wavelengths of radiation in any of the appropriate radiation bands as known in the art. Ellipsometry techniques typically measure changes in the polarization state of the reflected beam after interacting with the sample, while reflectometry techniques measure changes in the magnitude of the reflected beam. Scatterometry is a specific type of optical metrology that typically is used to measure diffraction, or optical scattering, of the probe beam due to the structural geometry of the sample, whereby details of the structure causing the diffraction can be determined.
These metrology techniques can be used to analyze a wide range of parameters, such as the thickness, crystallinity, composition, and refractive index of a film on a silicon wafer, for example. Measurements of this type can be made using reflectometry or ellipsometry techniques as described more fully in U.S. Pat. Nos. 5,910,842 and 5,798,837, each of which is hereby incorporated herein by reference. Other attributes of a sample that can be measured include critical dimensions (CD), line spacing, line width, wall depth, and wall profiles. Measurements of this type can be obtained using monochromatic scatterometry, such as is described in U.S. Pat. Nos. 4,710,642 and 5,164,790, each of which is hereby incorporated herein by reference. Another technique involves the use of broadband light to perform multiple wavelength spectroscopic reflectometry measurements. Examples of this approach can be found in U.S. Pat. Nos. 5,607,800; 5,867,276; and 5,963,329, each of which is hereby incorporated herein by reference. Other techniques utilize spectroscopic ellipsometric measurements, such as can be found in U.S. Pat. Nos. 5,739,909 and 6,483,580, each of which is hereby incorporated herein by reference.
When using one of these optical metrology techniques, it can be desirable to measure only a small region of a sample in a measurement box when there are a number of features and/or areas of different materials and/or concentration near the measurement box. A measurement box generally refers to a portion of the surface of a sample that is to be measured, and often is determined by structures and/or features of the sample. Using a large measurement spot could allow the measured signal to include multiple of these features and/or areas, which can be difficult to discern during signal analysis. For example, in order to measure the thickness of a film on a semiconductor wafer it can be necessary to utilize a measurement spot on the order of about 10 microns in order to avoid measuring features of nearby integrated circuits. Confining measurements to a small region is especially difficult for optical metrology devices that utilize probe beams at non-normal angles of incidence. In ellipsometers that can have incidence angles on the order of about 75°, for example, the high angle of incidence beams, of some diameter, project to a spot on the sample that has dimensions larger than the beam diameter. Also, at non-normal incidence, the deleterious effects of diffraction to enlarge the measurement spot, discussed below, can become more pronounced. Even for metrology systems that utilize near-normal angles of incidence the diffraction due to hard stops can cause difficulty in obtaining such small spot sizes.
FIG. 1 shows an exemplary optical metrology arrangement 100 of the prior art that can be used to capture ellipsometry and/or reflectometry data. The arrangement includes an illumination source 102 that creates a monochromatic or polychromatic probe beam 104. Any pinholes, apertures, or even optics of a finite size can comprise stops, which typically are used to control various aspects of the system. The probe beam. 104 is focused by one or more lenses 106 and/or other optical focusing elements to create an illumination spot on the surface of the sample 108 being examined. The illumination optics typically include an illumination pinhole 110 to help control the illuminated spot on the sample, and an illumination aperture 118 to help control the angular spectrum of the illumination. The angular spectrum refers to the distribution of light propagating at different angles with respect to a central ray of the system. The illumination aperture is typically at a focal plane of the illumination lens 106. The arrangement also can include at least one additional lens 112 and/or other optical focusing elements for collecting light reflected from the sample, in order to project the light onto the detector 114. A detection pinhole 116 and a detection aperture 120 can be used to select a portion of the light reflected from the illumination spot, which is to be projected onto the detector 114. If a laser is used as the illumination source, one or more of the pinholes and apertures on the collection and/or illumination sides may not be needed.
A goal of such a system is to allow the measurement spot to fall completely within the measurement box. The measurement spot refers to that portion of the surface from which light is reflected and subsequently received by the detector. The measurement spot can be controlled by aspects of the illumination and detection optics. The measurement box, in turn, typically is determined by structures or features of the sample, as the measurement box often is that portion of the surface of the sample that has a feature to be measured. In some cases, physical squares or boxes are manufactured onto the surface of a sample, such as a semiconductor wafer, in order to facilitate such measurements. In other cases, these boxes are merely conceptual regions. In general, there can be different measurement sensitivities at various locations within the measurement spot. A typical measurement spot does not have sharp edges or boundaries, but has a spatial distribution of measurement sensitivity. The spatial distribution of measurement sensitivity within the measurement spot can be affected by, for example, the illumination pinhole, the illumination aperture, the detection aperture, the detection pinhole, and aberrations in the optics. It can be desirable to have the measurement sensitivity as spatially confined as possible.
Chromatic aberration can limit the desired confinement of the measurement spot, and further can preclude use of a refractive optical element when using a broadband source. Reflective optics can be used to reduce chromatic aberration, but reflective optics may not be appropriate as the optics can partially polarize the beam and prevent, for example, accurate ellipsometry measurements. A significant problem that exists when using hard stops for pinholes and apertures is that the measurement spot will not have perfectly sharp edges, due to diffraction of the light. The measurement sensitivity will typically have a main lobe surrounded by secondary maxima, or broad areas of low-level sensitivity, sometimes referred to in the industry as “tails.” Such tails can arise from the illumination optics or from the detection optics.
A goal when designing a small spot instrument using an arrangement such as that shown in FIG. 1 is to minimize the ratio of light collected by the detector that is reflected from the area outside the measurement box versus the light reflected from inside the measurement box on the sample. One way to minimize this ratio in practice is to minimize the tails of the measurement sensitivity. The tails can be minimized in one approach by making the illumination and collection spots approximately the same size. Such a system is sometimes referred to as a confocal system. This is not always practical, however, as the instrument becomes extremely sensitive to focus and/or alignment errors. Such alignment can be difficult to maintain where the system experiences temperature, pressure, or other variations, or where the system requires shipping and/or movement. It therefore can be more practical to allow one of the illumination and collection spots to be larger than the other. While the smaller of the two spots can have the greatest influence on the measurement spot size, diffraction from both the illumination and collection optics still can have a significant effect on the tails of the measurement spot.
One technique that is used in the art to control the presence of tails is known as apodization. The McGraw-Hill Dictionary of Scientific and Technical Terms, 2nd Ed., 1985, by McGraw-Hill, Inc., defines “apodization” as “The modification of the amplitude transmittance of the aperture of an optical system so as to reduce or suppress the energy in the diffraction rings relative to that of the central Airy disk.” Apodization can be used to change the transmission characteristics of an aperture to attenuate the rings or tails. This can be done, for example, by replacing a traditional hard stop or the aperture with a tapered stop that has a gradual transition between transmissive and opaque regions. Diffraction typically arises from edges and sharp transitions, and a tapered stop will have a less well-defined edge. An optical system including such a tapered stop can produce a measurement spot with smaller tails. Apodization in optical metrology systems is discussed in U.S. Pat. No. 5,859,424, which is hereby incorporated herein by reference. For apodization, light is attenuated as a function of position on an apodizing filter containing a two-dimensional half-tone pattern, a pattern of alternating high transmittance areas and substantially opaque areas. While such an apodizing filter can be effective, the filter cannot be placed close to conjugates of the sample where the half-tone pattern of the filter would interfere with patterns on the sample. The filter is preferably placed near a focal plane of the optics or one of its conjugates, so the half-tone pattern does not project onto the sample. This placement constraint can be a disadvantage for various implementations and/or applications. Another problem with such apodizing filters is that in systems where a polarizer is close to the source, such as is described in U.S. Pat. No. 5,859,424, the apodizing filter must be placed at a position where the light is polarized. This can be problematic, as birefringence of the apodizing filter can disturb the polarization state. Another problem is that the apodizing filter works by blocking some of the light with which the filter is illuminated. To work properly, the entire half-tone pattern should be illuminated. The half-tone pattern therefore blocks some of the light with which it is illuminated in order to achieve the desired effect. Further still, a two-dimensional apodizing filter can be difficult to design and produce with regularity, can require precise alignment, and can be relatively expensive.